Occurrence of Metals and Concentration of Ores — Class 10 (English Medium) Science Notes

Natural Occurrence of Metals

Metals — found in the Earth's crust. Some in pure form, most as compounds.

Basic Terms — At a Glance

1. Minerals:

All natural compounds obtained from the earth.
Can be metallic or non-metallic.
Examples: limestone (CaCO₃), coal (carbon), rock salt (NaCl).

2. Ores:

Those minerals from which a metal can be profitably extracted in sufficient quantity.
(Not every mineral is an ore — but every ore is a mineral.)

Distinction — Ore vs Mineral

'Mineral' is a broader term. 'Ore' is narrower.

All ores → minerals, but all minerals → not ores.

Some Famous Ores

Sodium (Na):

Sodium chloride (sea salt): NaCl
Sodium carbonate: Na₂CO₃

Potassium (K):

Potassium chloride: KCl

Magnesium (Mg):

Magnesite: MgCO₃
Dolomite: CaCO₃·MgCO₃

Calcium (Ca):

Limestone: CaCO₃
Marble: CaCO₃

Aluminium (Al):

Bauxite: Al₂O₃·2H₂O ← Main ore!

Iron (Fe):

Hematite: Fe₂O₃ ← red-brown, main ore!
Magnetite: Fe₃O₄ ← magnetic!

Zinc (Zn):

Zinc blende: ZnS
Calamine: ZnCO₃

Copper (Cu):

Copper pyrites: CuFeS₂
Native copper (pure form)

Silver (Ag):

Native silver (pure form)
Argentite: Ag₂S

Gold (Au):

Native gold (pure form — in river sand, rocks)

Mercury (Hg):

Cinnabar: HgS

Froth flotation separating sulphide ore from gangue

Availability of Metals on Earth

Gold and Platinum — In Free State

Au, Pt — least reactive. Unaffected by air or water. Hence — found in pure form.

Known to humans for thousands of years:

Gold pieces directly from river sand.
Ancient civilizations — used gold in jewellery.

Silver and Copper — Partially

Ag, Cu — partly in pure form, partly as compounds.

Ag: Native silver, Argentite (Ag₂S) Cu: Native copper, Copper pyrites (CuFeS₂)

Iron, Magnesium, Sodium — Only as Compounds

These are more reactive — react quickly with air/water/others. Found only in compound form in nature.

Fe: Fe₂O₃, Fe₃O₄ (never pure) Mg: MgCO₃, MgCl₂ (never pure) Na: NaCl, Na₂CO₃ (never pure)

Relationship Between Reactivity and Occurrence

Reactivity	Occurrence	Examples
Very low	Free form	Au, Pt
Low	Partial free	Ag, Cu, Hg
Moderate	Only compounds	Pb, Zn, Fe
High	Only compounds	Mg, Al, Ca
Very high	Only compounds	Na, K

Principle: The higher the reactivity — the higher the chance of compound form.

Major Types of Ores

1. Sulphide Ores:

ZnS (zinc blende)
CuFeS₂ (copper pyrites)
HgS (cinnabar)
PbS (galena)

Generally for less reactive metals.

2. Oxide Ores:

Al₂O₃·2H₂O (bauxite)
Fe₂O₃ (hematite)
Fe₃O₄ (magnetite)
MnO₂ (pyrolusite)

For moderately reactive metals.

3. Carbonate Ores:

ZnCO₃ (calamine)
MgCO₃ (magnesite)
CaCO₃ (limestone)
FeCO₃ (siderite)

For many metals.

4. Halide Ores:

NaCl (rock salt)
KCl
CaF₂ (fluorspar)

For highly reactive metals.

Basic Steps of Extraction

Metal extraction = the process of obtaining pure metal from ore.

Three Main Steps

1. Concentration of ore:

Removing gangue (impurities) from ore.
Topic of this section.

2. Reduction of ore:

Obtaining metal from concentrated ore.
Topic of Section 7.

3. Refining of metal:

Making impure metal pure.
Topic of Section 8.

Flow Chart

  Mining
      ↓
  Ore obtained
      ↓
  Concentration (removing impurities)  ← Section 6
      ↓
  Reduction (obtaining metal)           ← Section 7
      ↓
  Refining (pure metal)                  ← Section 8
      ↓
  Pure metal — for use

Gangue — What is it?

The unwanted substances found along with the ore = Gangue.

Examples:

Clay
Sand
Pieces of stone
Other minerals (unwanted)

Purpose of concentration: To remove gangue and obtain pure ore.

Basic Methods of Concentration

Based on physical/chemical differences between ore and gangue:

Hand picking: visual difference.
Gravity separation: density difference.
Magnetic separation: magnetic difference.
Froth flotation: surface property difference.
Leaching: chemical difference.

We will mainly study the first four methods.

Four Main Methods of Concentration

1. Hand Picking

Selection by 'visible difference'.

When to use?

When ore and gangue are clearly different in appearance.
Such as: difference in colour, size, lustre.

Method:

Workers pick by hand.
Or by machines (modern).

Examples:

Identifying gold pieces.
Separating red hematite from clay.

Limitations:

Slow.
Expensive (labour).
For small quantities.

2. Gravity Separation

By difference in density.

Principle: Heavier ore — settles down, lighter gangue — flows away with water.

Method (Hydraulic Washing principle):

Crush the ore into fine powder.
Place on a sloping incline.
Pour a stream of water.
Lighter gangue — flows away with water.
Heavier ore — stays back.

Use:

Oxide ores — Hematite (Fe₂O₃) — heavier than gangue.
Gold ores.

By 'sieve' pan:

Ancient method of catching gold pieces in river sand.
Still used in some places today.

3. Magnetic Separation

Either ore or gangue — must be magnetic.

Principle:

Magnetic part — towards magnet.
Non-magnetic part — stays away.

Method:

Crush the ore into fine powder.
Place on a rotating conveyor belt.
At the end of the belt, an electromagnet.
Magnetic part — near the magnet, in one heap.
Non-magnetic part — in the heap further away.

Main Example:

Magnetite (Fe₃O₄):

Itself magnetic.
Gangue (clay, stone) — non-magnetic.
Separated by electromagnet.

Removing impure Fe from Tinstone (SnO₂):

Fe — magnetic.
SnO₂ — not.
Fe pulled out by electromagnet.

Pyrolusite (MnO₂):

Mn is magnetic.

Advantages:

Fast.
Automatic.
Accurate.

4. Froth Flotation Method

By difference in surface properties.

This method — for sulphide ores.

Main Examples:

ZnS (zinc blende)
PbS (galena)
CuFeS₂ (copper pyrites)

Principle: Sulphide ores — get wetted by oil (hydrophobic — not wetted by water). Gangue — gets wetted by water (hydrophilic).

Method (detailed):

Fine powder of the ore.
Add water in a large tank.
Add a little oil (pine oil).
Pass a strong current of air through the tank.
What happens:

Oil surrounds the ore particles.
Air creates froth (foam).
Bubbles in froth — bring ore particles up.
Gangue — settles at the bottom in water.

Collect the froth — dry — concentrated ore.

Auxiliary Chemicals:

Collector: pine oil, fatty acids — 'wets' the ore.
Frother: stabilises the foam.
Depressant: keeps unwanted ore at the bottom.

Example: Concentration of Zinc Blende (ZnS)

ZnS particles — up with froth.
Clay, stone — down in water.

Comparison and Choice of Methods

Which Method When?

Method	When to use?	Example
Hand picking	Visual difference	Gold pieces
Gravity	Density difference	Hematite (Fe₂O₃)
Magnetic	Magnetic difference	Magnetite (Fe₃O₄)
Froth flotation	Sulphide ores	ZnS, CuFeS₂, PbS

An Interesting Comparison

Two ores of Fe — two different methods!

Hematite (Fe₂O₃):

Non-magnetic.
Heavy.
→ Gravity separation.

Magnetite (Fe₃O₄):

Magnetic.
→ Magnetic separation.

That is — different ores of the same metal — different methods!

Leaching — 5th Method

Chemical method of concentration.

Principle: Dissolve the ore in a chemical solution — leaving the gangue behind.

Example: Leaching of Bauxite (Bayer Process):

Al₂O₃·2H₂O has impurities — Fe₂O₃, SiO₂.

1. Add bauxite to concentrated NaOH: $Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O$

Al has dissolved (NaAlO₂ — sodium aluminate). Fe₂O₃, SiO₂ — did not dissolve.

2. Filter:

Liquid: NaAlO₂
Solid: Fe₂O₃, SiO₂ (discarded)

3. Separate Al(OH)₃ from NaAlO₂: $NaAlO_2 + 2H_2O \rightarrow NaOH + Al(OH)_3$

4. Heat Al(OH)₃: $2Al(OH)_3 \xrightarrow{\Delta} Al_2O_3 + 3H_2O$

Now pure Al₂O₃ — ready for electrolysis.

After Concentration

Concentrated ore — now ready for next step: reduction. This is the topic of Section 7.

A Basic Rule

'Concentration method' is determined by the property of the ore. No single 'best' method — different for each ore.

[Board Important] Four main methods + a simple example = 5-mark question in board.

🧠 Memory Capsule

A quick glance just before the board exam.

1. Mineral vs Ore

Mineral: all natural compounds. Ore: those minerals from which metal can be profitably extracted.

2. Important Ores

Metal	Ore	Formula
Al	Bauxite	Al₂O₃·2H₂O
Fe	Hematite	Fe₂O₃
Fe	Magnetite	Fe₃O₄
Cu	Copper pyrites	CuFeS₂
Zn	Zinc blende	ZnS
Hg	Cinnabar	HgS
Pb	Galena	PbS
Na	Rock salt	NaCl

3. Occurrence Table

Reactivity	Form
Very low (Au, Pt)	Free
Low (Cu, Ag)	Partial
Moderate (Fe, Zn)	Compound
High (Mg, Na)	Only compound

4. 3 Steps of Extraction

Concentration (Section 6) — removing gangue
Reduction (Section 7) — obtaining metal
Refining (Section 8) — purification

5. 4 Methods of Concentration

Hand picking: visual difference (gold). Gravity: density difference (hematite). Magnetic: magnetic property (magnetite). Froth flotation: surface property (sulphide ores — ZnS, PbS, CuFeS₂).

6. Materials in Froth Flotation

Collector: pine oil
Ore up (in froth)
Gangue down (in water)

7. Bayer Process

For leaching of bauxite:

Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
Then Al(OH)₃ → Al₂O₃ + H₂O

8. Board's 'Golden' Questions

Difference between mineral and ore.
Description of froth flotation method.
When is magnetic separation useful?
Main ore of Al and its formula.
4 methods of concentration.

Final Formula: 'Concentration = removing gangue — choose method based on ore properties.'

Solved Examples

Example 1: NCERT — Mineral and Ore

What is the difference between mineral and ore? Explain with examples.

Solution:

Definitions

Mineral: All natural compounds found in the Earth's crust — whether metal can be extracted or not.

Ore: That mineral in which the metal is in such a sufficient quantity that it can be profitably extracted.

Main Differences

Basis	Mineral	Ore
Scope	Broader	Narrower
Metal content	May be low	Sufficient
Economic gain	Not necessary	Required
Examples	All rocks, soil	Only selected

Important Rule

Every ore → is a mineral. Every mineral → is not an ore.

An Interesting Example

Al — found in two minerals:

1. Bauxite (Al₂O₃·2H₂O):

Al content: 50-60%
Economically beneficial.
This is an ore.

2. Clay (ordinary clay):

Al content: 5-10% (alumino-silicate)
Very expensive to extract.
Not an ore, only a mineral.

That is — both contain Al, but only bauxite is called ore.

Other Examples

Ore vs Mineral:

Hematite (Fe₂O₃) — ore [for Fe extraction]
Magnetite (Fe₃O₄) — ore
Coal — mineral, not ore (Fe is not extracted from it)
Rock salt (NaCl) — both (ore for Na)

Final Definition

'Ore' = economically useful mineral.

This economic criterion changes with time — clay with low Al may also become an 'ore' if better technology is found in future.

[NCERT textbook — fundamental]

Example 2: NCERT — Major Ores

Write the major ores and their chemical formulas of the following metals:

(a) Al, (b) Fe, (c) Cu, (d) Zn, (e) Hg

Solution:

(a) Aluminium (Al)

Major Ore: Bauxite

Formula: $Al_2O_3 \cdot 2H_2O$ (hydrated aluminium oxide)

Al — most abundant metal in Earth's crust (~8%). In India: Odisha, Jharkhand, Gujarat.

(b) Iron (Fe)

Major Ores:

1. Hematite: $Fe_2O_3$

Red-brown colour.
Most important.

2. Magnetite: $Fe_3O_4$

Black colour.
Itself magnetic.

3. Siderite: $FeCO_3$

4. Iron pyrites: $FeS_2$ (Also known as 'fool's gold' — bright yellow, but iron ore.)

In India: Odisha, Jharkhand, Chhattisgarh, Karnataka.

(c) Copper (Cu)

Major Ores:

1. Copper pyrites: $CuFeS_2$ — most abundant. 2. Cuprite: $Cu_2O$ 3. Malachite: $CuCO_3 \cdot Cu(OH)_2$ — green. 4. Native copper — pure form, rare.

(d) Zinc (Zn)

Major Ores:

1. Zinc blende: $ZnS$ — most prominent. 2. Calamine: $ZnCO_3$ 3. Zincite: $ZnO$

In India: Rajasthan (Zawar Mines).

(e) Mercury (Hg)

Major Ore: Cinnabar

Formula: $HgS$ (bright red)

Spain (Almaden Mines) — most famous in the world.

Summary Table

Metal	Ore	Formula	Type
Al	Bauxite	Al₂O₃·2H₂O	Oxide (hydrated)
Fe	Hematite	Fe₂O₃	Oxide
Fe	Magnetite	Fe₃O₄	Oxide
Cu	Copper pyrites	CuFeS₂	Sulphide
Zn	Zinc blende	ZnS	Sulphide
Hg	Cinnabar	HgS	Sulphide

Note: Sulphide ores — for less reactive metals.

[NCERT textbook — important]

Example 3: NCERT — Hand Picking and Gravity Separation

When are hand picking and gravity separation methods useful? Explain with examples.

Solution:

Hand Picking

Principle: Visual difference — colour, size, lustre.

When useful? When there is a clear visual difference between ore and gangue.

Method:

Break the mineral into small pieces.
Pick by hand (or machine).
Ore pieces in one place, gangue in another.

Examples:

Picking gold pieces from river sand (visible yellow shine).
Separating hematite (red) from clay (brown).
Some coloured ores.

Advantages: simple, cheap. Disadvantages: slow, small quantities.

Gravity Separation (Hydraulic Washing)

Principle: Density difference. Heavier ore — lighter gangue.

When useful? When ore density is higher than gangue.

Method (detailed):

Crush ore to fine powder.
Place on a sloping metal plate / hydraulic chamber.
Pour stream of water.
What happens:

Light gangue — flows away with water.
Heavy ore — remains.

Collect the ore.

Examples:

Fe extraction from Hematite (Fe₂O₃):

Density of hematite ~5.3 g/cm³
Gangue (clay, sand): ~2.5 g/cm³
Fe₂O₃ heavier than water — settles.

Tinstone (SnO₂):

Density ~7 g/cm³
Gangue lower density.
Easily by gravity.

Comparison

Property	Hand Picking	Gravity
Basis	Visual	Density
Speed	Slow	Moderate
Use	Small scale	Large scale
Cost	High (labour)	Moderate
Example	Gold	Hematite

Key Insight

Both — old, traditional methods. Used as initial stage even in modern industry.

[Board: 3-5 marks]

Example 4: NCERT — Magnetic Separation

Describe the magnetic separation method with an example.

Solution:

Principle

Either ore or gangue — must be magnetic.

Then they can be separated by an electromagnet.

Method

Setup:

A conveyor belt (rotating moving belt).
Place fine powder of ore at one end of belt.
Electromagnet at the other end of belt.
Two heaps form below.

What happens:

Conveyor carries ore to magnet.
Magnetic part — pulled by magnet — falls in nearby heap.
Non-magnetic part — falls at end of conveyor — in farther heap.

Diagram (in words):

    Pour ore
    ↓
    Belt moves →→→→→→→ 
                     [Magnet]
                       |
                       |
              Non-magnetic    Magnetic
              (Far heap)     (Near heap)

Main Example

Magnetite (Fe₃O₄):

Fe₃O₄ — itself magnetic. Gangue (clay, sand) — non-magnetic.

Process:

Powder of ore + gangue on conveyor.
Fe₃O₄ — pulled towards magnet → concentrated ore.
Gangue — stayed away → discarded.

Other Examples

Removing Fe from Tinstone (SnO₂):

SnO₂ non-magnetic (main ore).
Fe (impurity) magnetic.
Pull Fe out with magnet — pure SnO₂ remains.

Pyrolusite (MnO₂):

MnO₂ weakly magnetic.
Separated with stronger magnet.

Wolframite (tungsten ore):

W slightly magnetic.
Can be separated.

When NOT to Use?

If both ore and gangue — non-magnetic. Then magnetic method is useless.

Example: Bauxite (Al₂O₃·2H₂O) — non-magnetic. Here leaching (Bayer) is useful.

Advantages

Fast.
Automatic.
Less labour.
Accurate.
Large scale.

[Board: 5-mark — with diagram]

Example 5: NCERT — Froth Flotation Method

Describe the froth flotation method in detail. For which type of ores is it used?

Solution:

For Whom?

This method — for sulphide ores.

Main Examples:

ZnS (zinc blende)
PbS (galena)
CuFeS₂ (copper pyrites)
HgS (cinnabar)

Basic Principle

Two types of substances:

Sulphide ores — get wetted by oil (hydrophobic — not wetted by water).
Gangue (clay, stone) — gets wetted by water (hydrophilic).

Method (Detailed)

Setup:

A large water-filled tank.
Air circulation at the bottom.
An agitator outside.

Steps:

1. Crush the ore to fine powder.

2. Add to tank — with water and some chemicals:

Pine Oil — Collector. 'Wets' the ore.
Frother — stabilises the foam.
Depressant — keeps unwanted ore down.

3. Pass strong air current into tank.

4. What happens:

Oil surrounds ore particles.
Air creates foam (froth).
Bubbles in froth — bring ore particles up.
Gangue — stays at bottom in water.

5. Collect the froth from the surface.

6. Dry the froth — concentrated ore is obtained.

Specific Example: Concentration of ZnS

Ore: ZnS + clay + sand + other impurities

Process:

Powder → tank.
Pine oil + water + air.
ZnS particles up with froth.
Clay, sand — down in water.
ZnS extracted from froth at the top.

Major Chemicals (Detailed)

1. Collector:

Pine oil (most common)
Sodium ethyl xanthate (Sodium ethyl xanthate)
Fatty acids

2. Frothers:

Pine oil
Cresylic acid

3. Depressant:

NaCN (Sodium cyanide) — keeps some ore at bottom.

4. Activator:

CuSO₄ — activates some inactive ore.

An Interesting Fact

PbS and ZnS together — how to separate?

Answer: Differential Froth Flotation. Add NaCN → ZnS stays down, PbS floats up. That is, selection by the same process.

Summary

Froth flotation — most widely used modern method. Sulphide ores around the world — by this method.

[NCERT — every year in board]

Example 6: NCERT — Types of Ores

How many main types of ores are there? Two examples for each.

Solution:

Main Types of Ores (4)

1. Oxide Ores

Formula: $M_xO_y$

Examples:

Hematite ( $Fe_2O_3$ ) — ore of Fe
Bauxite ( $Al_2O_3 \cdot 2H_2O$ ) — of Al
Magnetite ( $Fe_3O_4$ )
Cuprite ( $Cu_2O$ )
Zincite ( $ZnO$ )
Pyrolusite ( $MnO_2$ )

For most moderately reactive metals.

2. Sulphide Ores

Formula: $M_xS_y$

Examples:

Zinc blende ( $ZnS$ )
Galena ( $PbS$ )
Copper pyrites ( $CuFeS_2$ )
Cinnabar ( $HgS$ )
Argentite ( $Ag_2S$ )
Iron pyrites ( $FeS_2$ )

For most less reactive metals.

3. Carbonate Ores

Formula: $MCO_3$

Examples:

Limestone ( $CaCO_3$ )
Magnesite ( $MgCO_3$ )
Calamine ( $ZnCO_3$ )
Dolomite ( $CaCO_3 \cdot MgCO_3$ )
Siderite ( $FeCO_3$ )
Malachite ( $CuCO_3 \cdot Cu(OH)_2$ )

4. Halide Ores

Formula: $MX$ (X = F, Cl, Br, I)

Examples:

Rock salt ( $NaCl$ )
Sylvite ( $KCl$ )
Carnalite ( $KCl \cdot MgCl_2 \cdot 6H_2O$ )
Fluorspar ( $CaF_2$ )
Cryolite ( $Na_3AlF_6$ )

For most highly reactive metals.

Summary Table

Type	Formula	Famous Examples
Oxide	M_xO_y	Hematite, Bauxite
Sulphide	M_xS_y	Zinc blende, Galena
Carbonate	MCO₃	Limestone, Magnesite
Halide	MX	Rock salt, Cryolite

Reactivity-Ore Type Relationship

This is a 5-mark question:

Reactivity	Preferred Compound Type
High (Na, K, Ca)	Halide, Carbonate
Moderate (Al, Zn, Fe)	Oxide, Carbonate
Low (Cu, Hg, Pb)	Sulphide
Very low (Au, Ag, Pt)	Free form

This is an interesting principle — relationship between reactivity and ore type.

[Board: 5-mark — structured]

Example 7: NCERT — Bayer Process

How is pure Al₂O₃ extracted from bauxite? Explain with chemical reactions.

Solution:

The Basic Problem

Bauxite ( $Al_2O_3 \cdot 2H_2O$ ) has impurities:

Fe₂O₃ (iron oxide)
SiO₂ (silica)
TiO₂ (titanium oxide)

These impurities — must be removed before Al extraction.

Bayer Process — Steps

This is a method of leaching.

Step 1: Reaction with NaOH

Add fine powder of bauxite to concentrated NaOH at 150°C and high pressure.

Al₂O₃ — dissolves in NaOH:

$Al_2O_3(s) + 2NaOH(aq) \rightarrow 2NaAlO_2(aq) + H_2O$

(NaAlO₂ — sodium aluminate; soluble in water.)

Impurities — do not dissolve:

Fe₂O₃, TiO₂ — insoluble in NaOH.
SiO₂ — partially dissolves to Na₂SiO₃, but separated later.

Step 2: Filtration

Liquid NaAlO₂ — on one side. Solid impurities (Fe₂O₃, TiO₂, SiO₂) — on the other side.

Solid — discarded.

Step 3: Precipitation of Al(OH)₃

Add small Al(OH)₃ seed crystals to the NaAlO₂ liquid.

Cool and stir.

$NaAlO_2(aq) + 2H_2O \rightarrow NaOH(aq) + Al(OH)_3\downarrow$

(Al(OH)₃ — white precipitate; NaOH — in solution.)

Step 4: Calcination (heating)

Filter Al(OH)₃, wash, then heat at 1100°C.

$2Al(OH)_3 \xrightarrow{\Delta, 1100°C} Al_2O_3 + 3H_2O\uparrow$

Now pure Al₂O₃ (alumina).

Step 5: Electrolysis (Section 7)

Now Al₂O₃ → ready for Al extraction.

This is the 'Hall-Héroult' process.

Summary Diagram

Bauxite (impure Al₂O₃)
      ↓ + NaOH (concentrated, hot)
NaAlO₂ + insoluble impurities
      ↓ filter
NaAlO₂ solution
      ↓ + Al(OH)₃ seed crystal
Al(OH)₃ (precipitate)
      ↓ heat (1100°C)
Al₂O₃ (pure)
      ↓ electrolysis
Al (pure metal)

Importance

Bayer Process = Backbone of modern Al industry.

Millions of tonnes of Al₂O₃ each year — by this process.

[Board: 5-mark]

Example 8: A Mixed — Choose the Right Method

For the following ores, suggest the most suitable concentration method:

(a) Fe₃O₄ (Magnetite) (b) Fe₂O₃ (Hematite) (c) ZnS (Zinc Blende) (d) Native Au (pure gold) (e) Bauxite (Al₂O₃·2H₂O)

Solution:

(a) Fe₃O₄ (Magnetite)

Method: Magnetic separation.

Reason: Fe₃O₄ is itself magnetic. Gangue (clay, stone) non-magnetic.

Process: Conveyor belt + electromagnet.

(b) Fe₂O₃ (Hematite)

Method: Gravity separation.

Reason: Fe₂O₃ non-magnetic (unlike Fe₃O₄). But heavy (~5.3 g/cm³).

Process: Hydraulic washing with water stream.

Fe₂O₃ stays down, light clay flows away with water.

(c) ZnS (Zinc Blende)

Method: Froth flotation.

Reason: ZnS is a sulphide ore. Gets 'wetted' by oil.

Process: Water + pine oil + air. ZnS up with froth.

(d) Native Au (pure gold)

Method: Hand picking / gravity.

Reason: Au itself in pure form. Recognized by yellow shine.

Ancient: gold caught from river sand by sieve. Modern: cyanide leaching too.

(e) Bauxite (Al₂O₃·2H₂O)

Method: Chemical leaching (Bayer Process).

Reason: Al₂O₃ — dissolves in NaOH. Impurities (Fe₂O₃, SiO₂) — don't.

Process: With concentrated NaOH → NaAlO₂ → Al(OH)₃ → Al₂O₃.

Summary Table

Ore	Method	Principle
Fe₃O₄	Magnetic	Magnetic property
Fe₂O₃	Gravity	Density difference
ZnS	Froth flotation	Surface property
Native Au	Hand picking	Visual difference
Bauxite	Leaching	Chemical solubility

Principle

'Concentration method' is decided by the specific property of the ore.

Physical property → physical method. Chemical property → chemical method.

[Board: 5-mark — application]

Example 9: NCERT — Chemicals in Froth Flotation

Which chemicals are used in froth flotation method? Explain the role of each.

Solution:

Four Main Chemicals

1. Collector

Role: To make ore particles 'wet' by oil (i.e., wet with oil, not water).

Examples:

Pine oil (most common)
Sodium ethyl xanthate ( $C_2H_5OCSSNa$ )
Fatty acids

How does it work? A thin layer of oil forms around the ore particles. This layer — disliking water, liking air. Hence ore sticks to air bubbles.

2. Frother

Role: To make air bubbles stable.

Examples:

Pine oil (dual function)
Cresylic acid
Methyl isobutyl carbinol (MIBC)

How does it work? Forms a stable film on the surface of bubbles. Bubbles don't burst quickly — froth lasts.

3. Depressant

Role: To keep unwanted ore at the bottom (not let it come into froth).

Examples:

NaCN (Sodium cyanide) — most common

How does it work? Example: PbS and ZnS together — both are sulphides.

PbS to be separated.
NaCN makes ZnS particles 'water-loving'.
ZnS stays down, PbS comes up in froth.

This is Differential Flotation.

4. Activator

Role: To activate some inactive ore (so it can come into froth).

Examples:

$CuSO_4$ (copper sulphate)

How does it work? Example: To activate ZnS — add CuSO₄. Cu²⁺ deposits on ZnS surface. Now it gets wet by oil — comes up in froth.

Summary of the Process

Unwanted ore + Depressant → at bottom. Wanted ore + Collector + Activator (if needed) → in froth. Air bubbles + Frother → stable froth.

An Example — Separating Cu and Zn

Mixed ore: CuFeS₂ + ZnS + gangue

Step 1: NaCN (depressant) → ZnS down; CuFeS₂ up. Step 2: ZnS in separate tank → CuSO₄ (activator) → ZnS up.

That is, both ores separated by the same process.

Key Insight

Froth flotation — not a 'simple' method. Precise use of many chemicals. Modern chemical engineering.

[Board: 5-mark]

Example 10: Numerical — Metal in Ore

An ore has 80% Fe₂O₃. How much Fe is in 1000 kg of ore? (Fe=56, O=16)

Solution:

Step 1: Quantity of Fe₂O₃

1000 kg ore × 80% = 800 kg Fe₂O₃.

Step 2: Molecular Mass of Fe₂O₃

$Fe_2O_3 = (2 \times 56) + (3 \times 16) = 112 + 48 = 160$ g/mol.

% of Fe in Fe₂O₃: $\frac{2 \times 56}{160} \times 100 = \frac{112}{160} \times 100 = 70\%$

Step 3: Quantity of Fe

From 800 kg Fe₂O₃: Fe = 70% of 800 = 560 kg.

That is: $\text{Fe} = \frac{112}{160} \times 800 = 560 \text{ kg}$

Answer: In 1000 kg of ore, there will be 560 kg of Fe.

Additional Calculation

Total % Fe in ore: $\frac{560}{1000} \times 100 = 56\%$

That is, ore has 56% Fe.

(80% Fe₂O₃ × 70% Fe in Fe₂O₃ = 56%)

Practical Insight

The 'metal capacity' of an ore — fundamental measure of its value.

High-capacity ore — more valuable. Low-capacity ore — sometimes economically unviable.

A Comparison

% Fe in iron ores:

Hematite (Fe₂O₃) — 70% (pure)
Magnetite (Fe₃O₄) — 72% (pure)
Siderite (FeCO₃) — 48% (pure)
Iron pyrites (FeS₂) — 47% (pure)

In industry, those with higher Fe% — preferred.

[Board: 3-mark numerical]

Example 11: An Interesting — How is Gold Obtained?

In both ancient and modern methods.

Solution:

Gold — A Special Metal

Properties:

Very low reactivity.
Mostly in free form — river sand, rock crevices.
Sometimes with sulphide ores too.

Ancient Methods

1. Gravity (Panning)

Ancient method of catching gold pieces in river sand:

Method:

Take river sand in a sieve pan.
Wash with water.
Gold heavy — stays at bottom.
Light sand — flows away with water.
Gold left in the pan.

This method — still used in some places as 'gold panning'.

2. Hand Picking

Direct picking of bright yellow pieces.

Modern Method — Cyanide Leaching

This takes advantage of gold's very low reactivity.

Principle: Gold — forms a soluble compound with cyanide.

Step 1: Forming a compound of gold

$4Au + 8NaCN + O_2 + 2H_2O \rightarrow 4Na[Au(CN)_2] + 4NaOH$

(Sodium dicyanoaurate — soluble.)

Step 2: Filtration Liquid — gold (soluble). Solid — stones (discarded).

Step 3: Displacement of Au by Zn

$2Na[Au(CN)_2] + Zn \rightarrow Na_2[Zn(CN)_4] + 2Au\downarrow$

Au precipitate — in pure form.

This is the 'McArthur-Forrest' process.

Comparison

Method	Type	Time	Cost
Panning	Physical	Slow	Low
Hand picking	Physical	Slow	High (labour)
Cyanide	Chemical	Fast	Moderate (environmental damage)

Warning

NaCN — very toxic. Dangerous for health and environment.

Modern efforts — research on green alternatives (thiosulphate leaching).

An Interesting Fact

'Fool's gold' — Iron Pyrites (FeS₂). Looks like real gold — but much lighter. Real gold — 19.3 g/cm³ (very heavy). FeS₂ — ~5 g/cm³ (less heavy).

Against fraud — panning is very important!

[Board + General Knowledge]

Example 12: NCERT — Impurities in Ore

What are the unwanted impurities found in minerals called? Two examples.

Solution:

Definition

The unwanted impurities found along with ore — Gangue.

This English word 'Gangue' — comes from German.

Major Examples of Gangue

1. Clay:

Al-silicate compound.
With most ores.

2. Sand / Silica (SiO₂):

In all types of ores.
Common in Fe, Cu, Au ores.

Other examples of gangue:

Stone fragments
Other minerals (unwanted)
Organic matter
Calcium compounds

Why Remove Gangue?

1. Economic:

Transporting gangue — expensive (useless weight).
More energy in reduction.

2. Process:

Gangue — obstacle in reduction.
Affects purity.

3. Quality:

Impurity in final metal.
Weak metal.

Gangue vs Matrix

'Gangue' and 'matrix' — sometimes synonymous. Both = unwanted material along with ore.

Removing Gangue — Concentration

This is the main purpose of concentration.

Four methods (learnt earlier):

Hand picking
Gravity
Magnetic
Froth flotation

5th — chemical (leaching).

An Example — Bauxite

Ore: Al₂O₃·2H₂O

Gangue:

Fe₂O₃ (hematite)
SiO₂ (silica)
TiO₂

These impurities — removed by Bayer process.

Flux — An Interesting Element

Chemical added during reduction — to remove gangue.

Examples:

Acidic gangue (SiO₂) → basic flux (CaCO₃).
Basic gangue (CaO) → acidic flux (SiO₂).

Flux + Gangue = Slag.

This happens during reduction — covered in detail in Section 7.

Key Insight

'Gangue' = unwanted material in ore. Removal = concentration. Remaining during reduction = slag with flux.

[NCERT — fundamental question]

Example 13: A Logical — Identifying Inappropriate Method

Which of the following concentration methods is inappropriate for the given ore? Give reasons:

(a) Bauxite + magnetic separation (b) ZnS + gravity separation (c) Magnetite + froth flotation (d) Native Gold + leaching (cyanide)

Solution:

(a) Bauxite + Magnetic separation — Inappropriate ✗

Reason:

Bauxite (Al₂O₃·2H₂O) — non-magnetic.
Fe₂O₃ impurity also weakly magnetic.
No clear difference.

Suitable method: Chemical leaching (Bayer).

(b) ZnS + Gravity separation — Inappropriate ✗

Reason:

Density of ZnS ~4 g/cm³.
Gangue (clay, sand) — similar or close density.
Difference is insufficient.

Suitable method: Froth flotation.

(c) Magnetite + Froth flotation — Inappropriate ✗

Reason:

Fe₃O₄ — oxide ore, not sulphide.
Froth flotation is for sulphides.
Fe₃O₄ not 'wet' by oil — wet by water.

Suitable method: Magnetic separation (since itself magnetic).

(d) Native Gold + Leaching (cyanide) — Appropriate ✓

This is appropriate.

Reason:

Native Au — pure gold trapped in rock.
NaCN dissolves Au: $4Au + 8NaCN + O_2 + 2H_2O \rightarrow 4Na[Au(CN)_2] + 4NaOH$
Then Au precipitated by Zn.

This is the standard modern industrial method.

Summary

Ore + Method	Appropriate?	Correct method
Bauxite + Magnetic	✗	Bayer
ZnS + Gravity	✗	Froth flotation
Magnetite + Froth	✗	Magnetic
Au + Cyanide	✓	Same

Key Insight

Choice of concentration method — depends on specific properties of ore.

Three wrong combinations — all can be identified!

This is frequently asked in board.

[Board: 5-mark logical]

Example 14: NCERT — Ores and India

Main ores found in India and their geographic locations.

Solution:

India — A Mineral-Rich Country

For 60+ metals worldwide — sufficient ore in India.

Major Fe Ores

Hematite ( $Fe_2O_3$ ):

Odisha (Barbil-Koena region)
Jharkhand (Noamundi, Gua)
Chhattisgarh (Bailadila)
Karnataka (Bellary-Hospet)

Magnetite ( $Fe_3O_4$ ):

Karnataka (Kudremukh)
Tamil Nadu (Salem)
Andhra Pradesh

India — fourth-largest producer of Fe in the world.

Al — Bauxite

Bauxite ( $Al_2O_3 \cdot 2H_2O$ ):

Odisha (Koraput, Rayagada)
Jharkhand (Lohardaga)
Gujarat (Jamnagar)
Maharashtra (Kolhapur)
Andhra Pradesh

India — fifth-largest producer of Al in the world.

Cu — Copper Pyrites

Copper pyrites ( $CuFeS_2$ ):

Rajasthan (Khetri, Alwar)
Madhya Pradesh (Malanjkhand)
Jharkhand (Mosaboni)

Zn — Zinc Blende

Zinc blende ( $ZnS$ ):

Rajasthan (Zawar Mines, Rampura-Agucha)
Andhra Pradesh

India — largest producer of Zn in Asia.

Pb — Galena

Galena ( $PbS$ ):

Rajasthan (Zawar)
Andhra Pradesh Often along with Zn.

Au — Gold

Gold mines:

Karnataka (Kolar Gold Fields — KGF)
Karnataka (Hutti)
Andhra Pradesh (Ramagiri)

KGF — once one of the deepest mines in the world. Now closed, but historically important.

Ag — Silver

Argentite ( $Ag_2S$ ):

Rajasthan (mainly with zinc mines)
Karnataka

Mn — Manganese

Pyrolusite ( $MnO_2$ ):

Odisha
Karnataka
Madhya Pradesh

India — second-largest producer of Mn in the world.

Cr — Chromium

Chromite ( $FeCr_2O_4$ ):

Odisha (Sukinda Valley) — world-famous.

Summary Table

Ore	States
Hematite	Odisha, Jharkhand
Bauxite	Odisha, Gujarat
Cu pyrites	Rajasthan, Madhya Pradesh
Zinc blende	Rajasthan
Au	Karnataka
Mn	Odisha, Karnataka

Importance

Mineral sector — important in India's economic policy. Exports + domestic industry. 'Make in India' — from minerals to manufacturing.

[Board + General Knowledge]

Example 15: A Comparative — Two Ores, One Metal

Two ores of Fe — Hematite and Magnetite. Compare them and explain different concentration methods.

Solution:

Introduction to Both

Hematite:

Formula: $Fe_2O_3$
Colour: red-brown
Fe%: ~70%
Density: ~5.3 g/cm³
Magnetic: No

Magnetite:

Formula: $Fe_3O_4$ (= $FeO \cdot Fe_2O_3$ )
Colour: black
Fe%: ~72%
Density: ~5.2 g/cm³
Magnetic: Yes! (very strong)

Comparative Table

Property	Hematite	Magnetite
Formula	Fe₂O₃	Fe₃O₄
Colour	Red-brown	Black
Oxidation states	+3	+2 and +3
Fe%	70%	72%
Density	5.3 g/cm³	5.2 g/cm³
Magnetic?	Non-magnetic	Magnetic
Crystal	Trigonal	Cubic
Concentration method	Gravity	Magnetic

Difference in Concentration Methods

Hematite — Gravity Separation

Reason: Non-magnetic. But heavy (5.3 g/cm³).

Method:

Powder.
Hydraulic washing (with water stream).
Heavy Fe₂O₃ stays down; light clay flows away with water.

Magnetite — Magnetic Separation

Reason: Itself magnetic.

Method:

Powder.
Conveyor belt + electromagnet.
Fe₃O₄ — pulled towards magnet.
Gangue — at end of conveyor.

An Interesting Question

Can we separate Hematite by magnetic method?

Answer: No! Fe₂O₃ is non-magnetic.

Yes — if we first convert it to Fe₃O₄:

$3Fe_2O_3 \xrightarrow{\Delta, H_2} 2Fe_3O_4 + H_2O$

This is 'magnetic roasting' — done in some mines.

After Extraction

Concentrated ores of both → blast furnace. (Section 7)

$Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$ $Fe_3O_4 + 4CO \rightarrow 3Fe + 4CO_2$

Pure Fe from both.

Key Insight

'One metal — multiple ores — multiple methods.' Properties of ore = choice of method.

This is the art of 'Metallurgy'.

[Board: 5-mark comparative]

Example 16: A Concluding Question

(a) Difference between mineral and ore. (b) Concentration method for ZnS. (c) 2 examples of magnetic separation. (d) What are the 3 steps of extraction?

Solution:

(a) Mineral vs Ore

Basis	Mineral	Ore
Scope	Broader	Narrower
Metal	May be low	Sufficient
Economic	Any	Profitable
Examples	All rocks	Only selected

Rule: Every ore → mineral, but every mineral → not an ore.

(b) Concentration of ZnS — Froth Flotation

Principle: ZnS gets 'wet' by oil; gangue by water.

Method:

ZnS powder + water + pine oil → tank.
Pass air current.
ZnS coated by oil — floats up in froth.
Gangue — at bottom in water.
Collect froth from top — dry.

Chemicals:

Collector: pine oil
Depressant (if other ores): NaCN

(c) Examples of Magnetic Separation

1. Magnetite (Fe₃O₄):

Fe₃O₄ magnetic.
Clay, stone non-magnetic.
Fe₃O₄ pulled by electromagnet.

2. Removing Fe from Tinstone (SnO₂):

SnO₂ non-magnetic (main ore).
Fe (impurity) magnetic.
Pull Fe with magnet — pure SnO₂.

Others: Pyrolusite (MnO₂), Wolframite (W ore).

(d) 3 Steps of Extraction

Step 1: Concentration

Removing gangue from ore.
Methods: hand picking, gravity, magnetic, froth flotation, leaching.
Topic of Section 6.

Step 2: Reduction

Obtaining metal from concentrated ore.
Methods: with carbon, by electrolysis, by aluminothermite.
Topic of Section 7.

Step 3: Refining

Purifying impure metal.
Main method: electrolytic refining.
Topic of Section 8.

Flow Diagram

Mining → Concentration → Reduction → Refining → Pure metal

[Board: 5-mark mixed question]